1. Field of the Invention
The present invention relates to certain novel hydrazides and the use thereof as curing agents for epoxy resins.
2. Description of the Prior Art
Epoxy resins are widely employed as electric insulating materials, various moulded products, adhesives or coatings, because they give valuable cured resins having excellent mechanical, electrical and chemical properties when cured with suitable curing agents for example acid anhydride and amine curing agents. However, epoxy resin composition incorporating amine curing agents are cured rapidly at ordinary temperature and at elevated temperature and hence they lack storage stability. Also, epoxy resin composition incorporating acid anhydride curing agents are stable at ordinary temperature but heating for a long period of time at elevated temperature is required for full curing. Usually, tertiary amines, quaternary ammonium compounds or organo metal complexes are further added to the composition for the purpose of accelerating the curing rate. However, the addition of such cure accelerator impairs storage stability markedly.
There have been eagerly desired so-called latent curing agents which are compatible with epoxy resins to form a composition which is stable at relatively low temperature and which is rapidly cured when heated to an elevated temperature. Representative compounds which have been heretofore proposed as latent curing agents are dicyandiamide, dibasic acid hydrazide, boron trifluorideamine adduct, guanamine and melamine. Among these compounds, dicyandiamide, dibasic acid hydrazide and guanamine are useful in formulating epoxy resin compositions having excellent storage stability but full curing by means of these compound could be achieved only by heating at higher temperature than 150.degree. C. for a long time. Also, boron trifluoride-amine adduct is hard to treat owing to its high hygroscopic property and it affects adversely the physical properties of the cured resin.
There has been heretofore almost no latent epoxy curing agent which was known to cause rapid curing at moderately elevated temperatures, that is 100.degree. C.-150.degree. C. and which gives an epoxy resin composition having excellent storage stability at ordinary temperatures.